Dragging the flyout doesn't stop the scroll animation

[nimeratus]

moved from scratchfoundation/scratch-editor#298

Expected Behavior

After releasing the block palette, it stops moving

Actual Behavior

The block palette scrolls back to the start of the selected category

Steps to Reproduce

Steps to reproduce the behavior:

1. Go to 'https://scratch.mit.edu/projects/editor'
2. Click on a block category
3. While it's moving, start dragging the block palette
4. After releasing it, the block palette scrolls back to the block category

The bug doesn't happen if you use the scroll wheel for scrolling.

System Details

I've tested it in the browser. The bug is worse on mobile because there is no scroll wheel as workaround, but I can reproduce it on a computer with mouse, too

Screenshots

scratchbug.mp4